The present invention relates generally to apparatus for monitoring the penetration of a drill string into the earth during oil well drilling operations and, more particularly, but not by way of limitation, to apparatus for generating depth marker signals as drilling proceeds.
2. Brief description of the Prior Art
In the drilling of an oil well, the drill bit is mounted on the lower end of a drill string which is suspended from a drilling swivel, the drilling swivel, in turn, being suspended from a crown block at the top of the drilling rig so that the drilling swivel is lowered as the well is drilled. Such lowering of the drilling swivel is accomplished by paying out cable from a drum, such cable passing about sheaves on the crown block and drilling swivel to an anchor which fixes one end of the cable. The drilling proceeds in stages in each of which the drill string is lengthened by adding a segment of drill pipe to the upper end of the string and, thereafter, lowered to cause the drill bit to penetrate the earth an additional depth equal to the length of the added pipe segment. Specifically, in each stage, the drum is operated to raise the drilling swivel while the drill string is supported by the rotary table of the drilling rig; a new segment of the pipe is attached to the drilling swivel and connected to the upper end of the drill string; and the drum is then rotated, during drilling, to lower the drilling swivel and drill string. Thus, the drilling swivel undergoes repetitive vertical reciprications and the drum, similarly, undergoes corresponding, repetitive rotational reciprocations with the result that the depth of the well is incresed by the length of one segment of drill pipe in each reciprocation of the drilling swivel and drum.
It is common practice in the drilling of a well to monitor the rate at which the depth of the well increases and there are a variety of reasons for doing so. For example, should the drill string break, such fact will be indicated by a sudden decrease in the penetration rate. Similarly, changes in the penetration rate can be used to identify various strata where a well is drilled into a known geological formation so that, for example, the depth of such strata, at the well being drilled, can be determined. Similarly, penetration provides a qualitative measure of the state of wear of the drill bit.
A convenient way of monitoring the rate of penetration is to equip the well with a recorder having a steadily moving chart and provide a device which will mark the chart each time the depth of the well increses by some chosen increment of depth; for example, an increment of one foot. The spacings of the marks on different portions of the chart then indicate the relative rates of penetration for different depths into the well. A device which is particularly well-suited for making a record of the penetration of a drill string has been described in my U.S. Pat. No. 4,099,410 issued July 11, 1978. Such device has an input shaft which can be coupled to the drilling swivel by conventional means such that the input shaft will turn through one revolution as the drilling swivel is lowered through the selected depth increment. The device also includes a sensor assembly which generates a pneumatic signal each time the input shaft turns through one revolution and a control circuit which receives the pneumatic signals generated by the sensor assembly and converts such signals into mechanical movements of a rod which can be attached to the pen of a strip chart recorder.
It will be recognized that the length of drill string hoisting cable paid out by the drum in each rotational reciprocation thereof also provides a measure of drill string penetration and there are advantages to be obtained in operating a device, such as that disclosed in my aforementioned U.S. Pat. No. 4,099,410, from movements of the cable. For example, since the cable extends to the drum which is placed beside the derrick of a drilling rig, so that there is no need for any portion of the penetration rate recording system to be located on the derrick, the connection of such recording system to the drilling rig can be simplified and possible entanglement of cables, pneumatic tubing and the like, forming a part of the system, can be largely avoided. However, attempts to use movement of the drill string hoisting cable as a measure of the movement of the drill string encounter serious difficulties. The cable is subject to whipping movements so that attachment of a sensor to the cable can be a severe problem. While this problem can be overcome by coupling the system to the drum upon which the cable is wrapped, so that angles of rotation of the drum are used as measures of drill bit penetration, such coupling has, in the past, given rise to another problem. The cable is wrapped on the drum in laps and, as a practical matter, it often occurs that portions of two laps will be paid out from the drum as the drill string is lowered through a distance equal to the length of a pipe segment. Since these laps will have different diameters, the relationship between angles through which the drum turns and distances the drill string is lowered will vary from lap to lap. As a result, successive marks made on a chart by a device interposed between the drum and a recorder will sometimes correspond to one penetration distance and, at other times, correspond to a different penetration distance so that the chart may very well be misinterpreted. At the very least, the chart would become difficult to interpret.